1. Field of the Invention
The present invention relates to a magnetic recording device and a magnetic recording method, and more particularly to a magnetic recording device and a magnetic recording method using a microwave-assisted magnetic recording function for recording signals to a magnetic recording medium.
2. Description of the Related Art
In accordance with the increase of a recording density of a magnetic recording-reproducing device, which is represented by a magnetic disk driving device, the minuteness of bit cells of digital data is increased, the data being recorded to a magnetic recording medium. As a result, fluctuations of signals occur due to heat fluctuation, the signals being detected by a recording head element of a thin film magnetic head, and a signal to noise (S/N) ratio deteriorates. In the worst case, signals may disappear.
In such situation, in a magnetic recording medium that is used for a perpendicular magnetic recording system, which has been practically used in recent years, it is effective to increase perpendicular magnetic anisotropy energy Ku of a magnetic recording layer configuring the magnetic recording medium. On the other hand, a thermal stability index S, which corresponds to heat fluctuation, is expressed by a following expression, and it is said that the thermal stability index S should be normally 50 or more.S=Ku·V/kB·T  (1)Ku: perpendicular magnetic anisotropy energy, V: volume of grains configuring a magnetic recording layer, KB: Boltzmann constant, T: absolute temperature
According to the Stoner-Wohlfarth model, an anisotropy magnetic field Hk and coercive force Hc of the magnetic recording layer are expressed by a following expression. With the Ku increasing, the coercive force Hc increases (note, Hk>Hc in a normal magnetic recording layer).H=Hc=2Ku/Ms  (2)Ms: saturation magnetization of a magnetic recording layer
In order to perform a magnetization reversal of a magnetic recording layer that corresponds to a predetermined data series, a recording head element of a thin film magnetic head needs to apply a recording magnetic field having a steepness that is almost the same as that of an anisotropy magnetic field Hk of the magnetic recording layer. In a magnetic disk driving device that has been practiced using a perpendicular magnetic recording system, a recording head element using a single magnetic pole is used, and a recording magnetic field is applied in a perpendicular direction from an air bearing surface (ABS) thereof to a magnetic recording layer. The intensity of the perpendicular recording magnetic field is proportional to a saturation magnetic flux density Bs of a soft magnetic material that forms the single magnetic pole, and therefore a material having a high saturation magnetic flux density Bs has been developed and practiced. However, according to the Slater-Pauling curve, the saturation magnetic flux density Bs has a practical upper limitation when Bs=2.4 Tesla (T), so that the current situation has approached the practical limitation. A thickness and a width of a currently-used single magnetic pole is approximately 40-80 nm. However, in order to increase the recording density, the thickness and the width need to be decreased. With the decrease, a generated perpendicular magnetic field further decreases.
In order to resolve such problems, various technologies have ever been proposed. As one of those, a technology is known that applies an alternate-current (AC) magnetic field in addition to a regular recording magnetic field from a magnetic head, and the AC magnetic field has a frequency that corresponds to a medium magnetic resonance frequency in a state where a recording magnetic field is applied. When a frequency of the AC magnetic field is close to a magnetic resonance frequency of magnetic particles that configures a medium in a state where a recording magnetic field is applied, precession movement of spins is excited, thereby causing a magnetization reversal. The present technology theoretically uses this phenomenon. By applying such AC magnetic field, recording to a medium material having large Ku can be easily performed as attempting to decrease a recording magnetic field. Because a frequency of the applied AC magnetic field is in a microwave band of approximately several—several tens GHz, this technology is referred to as microwave-assisted magnetic recording. Hereinafter, the above-described AC magnetic field is referred to as a microwave magnetic field. A numeral simulation has validated that, when the technology is applied to the perpendicular magnetic recording, the significant decrease of the recording magnetic field is achieved and a high density recording becomes possible. Jian-Gang Zhu, Xiaochun Zhu, and Yuhui Tang, ‘Microwave Assisted Magnetic Recording’ IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 1, JANUARY 2008 should be referred.
In the microwave-assisted magnetic recording, a system is known that supplies microwave signals (power) to a microwave generating element, the microwave signals being supplied from a microwave signal generating circuit that is formed independently from the magnetic head. The above-described system is called separate excitation system microwave-assisted magnetic recording. In this system, microwave signals (power) are supplied to the microwave generating element formed in a front end of the magnetic head via a microwave transmission line. JP Laid-Open Patent Application No. 2010-003339 discloses such a microwave transmission line and also discloses that a recording magnetic field for magnetic recording can be reduced by combining with a single magnetic pole type magnetic head.
However, JP Laid-Open Patent Application No. 2010-003339 doesn't disclose anything regarding application time duration of a microwave magnetic field, which is necessary for the magnetization reversal. When a microwave generating element driving current is continuously applied to the microwave generating element for the purpose of continuous application of microwave, consumption power of the microwave generating element becomes large. Also, when the consumption power of the microwave generating element is large, the microwave generating element may be fused due to heat generation of the element. Even when such a fusion does not occur, an air bearing surface may deform due to the heat, thereby allowing a flying state of the magnetic head to vary.